Minimally invasive posterolateral fusion

ABSTRACT

A percutaneous system in which tubular retractors are utilized as a means of access, visualization, and a controlled orientation for preparation of posterior spinal elements and delivery of graft material for posterior spinal fusion. The tubular retractors are affixed to screw extensions to rotate and articulate relative to multi-axial screws percutaneously inserted into respective pedicles of vertebral bodies. The procedure is done bilaterally, although it could be performed unilaterally in cases where the anatomy or surgeon preference dictates. A number of different deployable graft concepts are disclosed, the primary features being to allow for creation of a graft space volume and a means for delivery of the graft material preferred by the surgeon.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/016,743, filed Jun. 25, 2014, the entire contents ofwhich are incorporated by reference herein.

FIELD OF THE INVENTION

The subject invention relates generally to the field of minimallyinvasive spinal fusion and more particularly to a minimally invasivemethod for achieving a posterolateral spinal fusion. The subjecttechnique may be used as an augment for a minimally invasive interbodyspinal fusion. It may also be helpful as a sole fusion site for a distallevel fusion during cases where interbody fusion may not be achievable,for example in long deformity fusions or very challenging spinalanatomy.

BACKGROUND OF THE INVENTION

Current methods of minimally invasive spinal fusion primarily focus onthe interbody space and use of an interbody spinal implant with bonegrafting inside the intervertebral disc space. Traditionally, surgeonshave performed an open procedure for spinal fusion that involves boththe interbody fusion and a posterior lateral fusion. The posteriorlateral fusion had been performed an open manner with tissue retractionand direct visualization of the posterior elements including the midlineelements of the facet and pars intra-articularis, as well as the morelateral elements of the intertransverse process. With the advent ofminimally invasive spinal fusion, direct access and visualization of themidline and lateral elements of the vertebral bodies have been difficultto perform in a minimally invasive manner, while access to the disc hasbeen done with tubular retractors dilating tissues and muscle layers.

A number of investigators have proposed using tubular access to theposterior elements for percutaneous delivery of graft material. Howeverthe anchor points for these tubular elements have generally been of afixed nature and do not allow for the multiaxial motion of the deliverysystem needed for broad access and visualization of the midline andlateral bony elements. For example, Oktavec et al. in U.S. PatentApplication Publication No. 2012/0253316, entitled “PercutaneousBiologic Delivery System”, filed Mar. 28, 2012, disclose such a fixedanchor point where the pivot points are distal from the fixed pin basedanchor point and therefore have limited mobility. In addition, thepivoting element is relatively distant from the anchor element, whichwill tend to result in a more significant retraction of the surroundingtissues and muscle.

Accordingly, there is a need for an improved minimally invasive spinalfusion procedure that will provide for more efficient and desiredpreparation of the surrounding bony surfaces prior to delivery of animplant and bone graft materials.

SUMMARY OF THE INVENTION

The system of the subject invention provides for minimally invasiveaccess to the posterior spinal elements necessary for bony preparationand graft delivery during posterolateral spinal fusion. A series oftubular retractors is fixed percutaneously to multiaxial screwextensions to provide an access pathway down to the bony elements of theposterior spine. Visualization is achieved through the tubularretractors using either direct vision or endoscopy or microscopemagnification. In addition, slots provided along the screw extensionsand windows in the retractors provide for additional access pointsseparate from the point of visualization through the tubular retractor.By anchoring the tubular retractors on the multiaxial spinal screw, thesurgeon is provided with a well-known access and anchorage point (thespinal pedicle) and the ability to pivot and rotate the tubularretractors as necessary for visualization, bony element preparation, andgraft delivery for spinal fusion. An injectable bone graft substitutematerial such as the geneX® synthetic bone graft (Biocomposites Ltd,Keele, UK) may be delivered directly at the graft site or along theelements spanning two screw-based access points. The tubular retractorhas been designed for balancing visualization with minimizing necessarytissue dilation or retraction, as well as for providing a controlled andknown access corridor to the posterior spinal elements. Methods forpreparing a graft channel between the spinal elements along the midlineprocesses or lateral transverse processes are also provided.

DESCRIPTION OF THE FIGURES

FIG. 1 is a top perspective view of a portion of a spine illustratingpartially assembled components a minimally invasive spinal fusion systemincluding a first embodiment of a tubular retractor of the invention.

FIG. 2 is the view of FIG. 1 with the components of the minimallyinvasive spinal fusion system assembled and attached to the spine.

FIG. 3 is a perspective view of a second embodiment of a tubularretractor.

FIG. 4 is a side elevation view of the tubular retractor of FIG. 3.

FIG. 5 is top perspective view of a third embodiment of the tubularretractor attached to the spine.

FIG. 6 is top perspective view of the tubular retractor of FIG. 5 in usewith a tissue preparation instrument.

FIG. 7 is an enlarged top view of FIG. 6 illustrating the visualizationof the access port of the tubular retractor as would be seen by asurgeon.

FIG. 8 is a perspective view of a third embodiment of a tubularretractor.

FIG. 9 is top perspective view of the tubular retractor of FIG. 8 in usewith a blunt tissue dilator introducer.

FIG. 10 is an enlarged top view of FIG. 9 illustrating the blunt dilatorextending through an access port of the tubular retractor as visualizedby a surgeon through the access port.

FIG. 11 is a perspective view of a fourth embodiment of a tubularretractor.

FIG. 12 is a top perspective view of a first tissue dilator beinginserted through tissue of a patient adjacent to a pedicle screwextension assembly attached to the spine.

FIG. 13 is a top perspective view of a second tissue dilator beinginserted through tissue of a patient adjacent to the pedicle screwextension assembly attached to the spine.

FIG. 14 is view of tubular retractor of the second embodiment shown inFIG. 3 slidingly attached to the second tissue dilator in FIG. 13.

FIG. 15 is a side exploded elevation view of the pedicle screw extensionassembly, tissue dilator and retractor as shown in FIG. 13.

FIG. 16 is a further top perspective view of FIG. 13 showing a secondpedicle screw extension assembly extending through tissue of thepatient.

FIG. 17 is a further top perspective view of FIG. 16 with the tissuedilator removed and a second tubular retractor placed over the secondpedicle screw extension assembly.

FIG. 18 is an enlarged top view of FIG. 17 is visualized by a surgeon tothe access ports of the first and second tubular retractors.

FIG. 19 is a top perspective view of FIG. 17 showing use of anelectrocautery device placed directly through a side window of onetubular retractor.

FIG. 20 is a top perspective view of FIG. 17 showing a powered drillplaced directly through the pathway of the access port of one tubularretractor.

FIG. 21 is a top perspective view of FIG. 17 showing use of a powerdrill placed directly through a side window of one tubular retractor.

FIG. 22 is a top perspective view of FIG. 17 showing a blunt dilatorattached to and introducer with the blunt dilator being placed throughside opening window of one tubular retractor.

FIG. 23 is an enlarged top perspective view of FIG. 17 showing placementof a hollow perforated graft between spinal elements.

FIG. 24 is a view similar to the view of FIG. 23 showing an alternativeperforated tube placed between spinal elements for graft materialdelivery.

FIG. 25 is a view similar to the view of FIG. 23 showing an alternativemanufactured perforated anatomical implant followed by injection offlowable biomaterial.

FIG. 26 a is a view similar to the view of FIG. 23 showing analternative flexible and perforated tubular implant being pushed along acurved rigid access tube.

FIG. 27 is a view similar to the view of FIG. 26 with tubular retractorsremoved for clarity and with the curved rigid tube in partialcross-section to reveal the tubular implant in the process of beingdelivered to the spine.

FIG. 28 is a further view of FIG. 27 with tubular retractors in placeshowing additional bone graft material being injected around the tubularimplant.

FIG. 29 is a further view of FIG. 28 showing some of the injected graftmaterial surrounding the previously placed tubular implant.

FIG. 30 is a perspective view of a self-contained delivery device forholding and delivering the tubular implant of FIG. 26.

FIG. 31 is perspective view of another device for more controlleddelivery of the tubular implant of FIG. 26.

FIG. 32 is a further view of the delivery device of FIG. 31 showing thedistal end of an outer sheath of the delivery device extending through aside window of a tubular retractor.

FIGS. 33 and 34 show the process of withdrawing the outer sheath of thedelivery device of FIG. 32 and leaving the tubular implant behind on thespinal elements.

FIG. 35 is an enlarged perspective view of the tubular implant of FIG.34 after being left behind on the spinal elements.

FIG. 36 is a top perspective view of a pair of bilateral tubularimplants and connecting rods having been percutaneously placed on thespine.

FIG. 37 is a top perspective view of a threaded screw tap as analternative access point to the spinal elements prior to placement of amulti-axial screw.

FIG. 38 is a top perspective view similar to FIG. 2, showing a modifiedretractor rotatably attached to the tap of FIG. 37.

FIG. 39 shows a top view of the tap and tubular retractor of FIG. 38depicting the access port as would be visualized by a surgeon.

DESCRIPTION OF THE EMBODIMENTS

For the purposes of promoting and understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

As set forth herein, the subject invention provides the ability tovisualize and access surrounding bony elements and soft tissues in amobile manner while utilizing a well-established anchor point in thepedicle of spinal vertebrae. Instrumentation necessary for soft tissuepreparation and bony removal may be placed either directly through theaccess port, through access slots in the side of the tubular retractor,or through slotted elements in the screw extension. This instrumentationprovides for efficient and desired preparation of the surroundingtissues prior to delivery of an implant and bone graft materials. Thesystem disclosed hereinbelow for establishing a pathway between adjacentvertebrae comprise a series of blunt dissectors or dilators that maythen be followed by a deployable graft implant.

One aspect of the system described herein is in allowing a surgeon toperform a posterior lateral fusion using minimally invasive access andvisualization techniques such as a fixed and mobile tubular retractor.In general, the instrumentation and method allows for safe accessthrough the overlying tissue and muscle, direct visualization of thebony elements (e.g. the preparation of the fusion bed), and delivery ofbone graft material between multiple spinal segments necessary toachieve spinal fusion. As is well known, a spinal segment comprisesopposing vertebral bodies of a spine and the intervertebral disctherebetween.

Turning now to FIGS. 1-2, details of one particular arrangement of aminimally invasive spinal fusion system 1 comprising a pedicle screwextension assembly 10 and a tubular retractor 100 for minimally invasivespine fusion are described. Pedicle screw extension assembly 10comprises a spinal screw 12 and an elongate screw extension 14 releasblyattached to screw 12. Spinal screw is an elongate fixation member,preferably a multi-axial pedicle screw attached as shown to a pedicle 16of a vertebral body 18 of a spine 20. Elongate extension 14 defines alongitudinal axis 22 extending generally centrally along the length ofextension 14. Extension 14 is generally cylindrical along its outerlength and has a pair of flat diametrically opposed surfaces 24extending lengthwise. A pair of substantially opposite slots 26 extendsthrough flat surfaces 24, each slot 26 communicating with a lumen 28extending generally centrally through extension 14 along 20. Pediclescrew 12 includes a threaded shaft 12 a and a yoke 12 b articulatinglyand rotatably attached to said screw shaft 12 a (see FIG. 15). Elongatescrew extension 14 is releasably coupled to yoke 12 b for articulationand rotation of extension 14 about screw shaft 12 a. Details of pediclescrew extension assembly 10 and the arrangement by which elongateextension 14 is releasably coupled to yoke 12 b are described more fullyin U.S. Pat. No. 8,845,640, entitled “Pedicle Screw Extension for Use inPercutaneous Spinal Fixation”, issued in the name of Scott Mclean et al.on Sep. 30, 2014 (the '640 patent). The '640 patent is assigned to thesame assignee as is the subject application, the contents of the '640patent being incorporated herein by reference in their entirety.

Referring still to FIGS. 1-2, tubular retractor 100 provides for aclosed channel access to the facet region 30 of spine 20, tubularretractor 100 being releasably affixed to the multiaxial pedicle screw12 through extension 14. Tubular retractor 100 is slid over extension 14and pushed down through an incision formed through the skin in order toabut against the posterior spinal elements of spine 20. Instruments, aswill be described, may be passed through the closed tubular workingspace in order to access the facet capsule and adjacent midline elementsof spine 20.

Tubular retractor 100 is preferably elongate having a distal end 100 a,a proximal end 100 b and an axial length therebetween. Tubular retractor100 includes an access port 102 including a partially cylindrical wall103 defining a partially cylindrical pathway 104 extending substantiallythe entire length of tubular retractor 100, pathway 104 being configuredfor instrument access and visualization by the surgeon. Pathway 104defines a second longitudinal axis 106 extending along the length oftubular retractor 100 through distal end 100 a and proximal end 100 b.Tubular retractor 100 includes a pair of generally circular attachmentrings 108 a and 108 b disposed respectively at the distal end 100 a andproximal end 100 b. Rings 108 a and 108 b are axially spaced from eachother and together define an attachment portion for attaching tubularretractor 100 to elongate extension 14, the axial length of the combinedrings 108 a and 108 b being different than and less than the length ofaccess port 102. Each of rings 108 a and 108 b has a curved interiorsurface for relatively close sliding fit over the generally cylindricalouter surface of extension 14. An interior surface 112 of each of rings108 a and 108 b is formed as a flat surface to provide a cooperativekeying arrangement with one of flat surfaces 24 of extension 14, as willbe described. Rings 108 a and 108 b together define a third longitudinalaxis 110 that is generally parallel to and offset from secondlongitudinal axis 106. The open axial space 108 c between rings 108 aand 108 b defines a side opening of tubular retractor 100 that is incommunication with pathway 104.

FIG. 2 demonstrates the attachment of tubular retractor 100 to screwextension 14, which is placed down through the same incision of the skinof the patient through which pedicle screw extension assembly 10extends. During attachment, flat surfaces 112 of rings 108 a and 108 bslidingly engage flat surfaces 24 and extension 14 releasably fixingtubular retractor 100 to extension 14 in a manner to prevent relativerotation between tubular retractor 100 and screw extension 14 whileallowing relative axial movement therebetween. Thus, joint articulationand rotation of tubular retractor 100 relative to threaded shaft 12 a ofpedicle screw 12 is provided. During attachment third longitudinal axis110 of rings 108 a and 108 b is substantially coaxial with firstlongitudinal axis 22 of screw extension 14 with second longitudinal axis106 of pathway 104 being laterally offset from first longitudinal axis22. As such, pathway 104 is laterally offset from screw extension 14 androtatable and articulatable with screw extension 14 about spinal screwshaft 12 a. As depicted in FIG. 2, tubular retractor 100 is fully seatedwith access port 102 defining a closed working channel pathway 104having a fixed area transverse to second longitudinal axis 106 for safeaccess to spinal facets and other posterior spinal elements. Instrumentscan be placed through pathway 104 to remove soft tissues overlying bonyelements of the posterior portion of spine 20, followed by decorticationof bony surfaces and placement of a suitable bone graft, as will bedescribed. In addition, tubular retractor 100 can be rotated laterallyabout first longitudinal axis 22 for access to the transverse processes32 of the spine 20. Upon rotation of tubular retractor 100 about firstlongitudinal axis 22 of extension 14 an area of bony surface of spine 20is exposed adjacent pedicle screw 12 that is greater than the fixed areaof said pathway 104. Further, pathway 104 may be accessed through sideopening 108 c via slots 26 extending through screw extension 14. Tissuepreparation instruments or bone graft material devices may be introducedthrough side opening 108 c, through slots 26 and into pathway 104 whilethe surgeon has full access to and visualization of the prepared sitethrough pathway 104 to observe the tissue preparation or bone graftmaterial delivery.

Turning now to FIGS. 3-4, a second alternative tubular retractor 200 foruse in minimally invasive spinal fusion system 1 is described. Tubularretractor 200 is elongate having a distal end 200 a and a proximal end200 b and overall length L₁ extending from distal end 200 a to proximalend 200 b as illustrated in FIG. 4. Tubular retractor 200 comprises anaccess port 202 and an attachment portion 204. Attachment portion 204extends longitudinally over the length L₁ and access port 202 extendsfrom distal end 200 a for a length L₂ along tubular retractor 200 towardproximal end 200 b. Length L₂ of access port 202 is different than andless than the length L₁ of attachment portion 204. Length L₁ is adaptedto the amount of soft tissue and muscle overlying the spinal elementsand necessary to safely reach the site of bone fusion procedure.Attachment portion 204 comprises a generally curved open channel 204 aand defines a longitudinal attachment axis 206 similar to thirdlongitudinal axis 110 defined by attachment rings 108 a and 108 b oftubular retractor 100. Attachment portion 204 has a curved interiorsurface 204 b for relatively close sliding fit over the generallycylindrical outer surface of screw extension 14 as illustrated in FIG.3. An interior surface 204 c of attachment portion 204 is formed as aflat surface as depicted in FIG. 3 to provide a cooperative keyingarrangement with one of flat surfaces 24 of extension 14, as describedhereinabove with respect to tubular retractor 100.

Access port 202 includes a partially cylindrical wall 208 defining asubstantially enclosed, partially cylindrical pathway 210 extending thelength L₂ of access port 202, pathway 210 being configured forinstrument access and visualization by the surgeon. Pathway 210 definesa longitudinal access axis 212 similar to second longitudinal axis 106defined by access port 102 of tubular retractor 100. Longitudinal accessaxis 212 is generally parallel to and offset from longitudinalattachment axis 206. Access port 202 has a pair of side openings definedby windows 214 extending through two substantially opposing sides ofwall 208 in communication with pathway 210, windows 214 being configuredfor placement of instruments and implants, as well as for potentiallyenhanced visualization and illumination.

Pathway 210 of access port 202 has a dimension transverse tolongitudinal access axis 212, such as diameter D, as shown in FIG. 3,defining pathway 210 as a fixed, substantially enclosed area for safeaccess to spinal facets and other posterior spinal elements. Instrumentscan be placed through pathway 210 to remove soft tissues overlying bonyelements of the posterior portion of spine 20, followed by decorticationof bony surfaces and placement of a suitable bone graft, as will bedescribed.

The tubular retractors 200 range in sizes adaptable to patient anatomyvariations. The diameter D and the length L₂ of access port 202 may varybased on patient anatomy. A surgeon may use different lengths ordifferent diameters depending on the curvature of the spine and amountof overlying soft tissues and muscle. The configuration of tubulartractors 200 as shown in FIG. 3 demonstrates a diameter D ranging from18 mm to 22 mm, with lengths L₂ from 4 cm up to 8 cm. A kit of differentsized tubular retractors 200 and various instruments, dilators and graftimplants described hereinbelow may be provided for a surgical procedure.The surgeon may select an access port length L₂ to be as short aspossible (just exiting the skin line, S in FIG. 16), in order tomaximize the available angular trajectory that may be used forvisualization and placement of instruments such as rongeurs or drillsfor bone decortication. In addition, the presence of side access windows214 provides for an alternate way to place instruments in a trajectorydifferent from the line-of-sight of the surgeon through pathway 210. Atab 216 projects from wall 208 that allows the surgeon to optionallyaffix tubular retractor 200 to an operating room table (via a flexiblearm assembly not shown) for additional stabilization or for hands-freeoperation through access port 202.

Turning now to FIGS. 5-7, a third alternative tubular retractor 300 foruse in minimally invasive spinal fusion system 1 is described. Tubularretractor 300 includes a modification of tubular retractor 200.Reference numerals illustrating elements of tubular retractor 300 thatare common with tubular retractor 200 are increased by 100 for ease ofdescription. In tubular retractor 300, an angular side access port 318having a guide channel 320 extends from attachment portion 304. A sideopening defined by an angular side access channel 320 intersectslongitudinal attachment axis 306 of attachment portion 304 andcommunicates through the slots 26 and lumen 28 of extension 14 withpathway 310 of access port 302 to allow for angled access to the pathway310 across the screw extension 14. This allows for placement of aninstrument 322 such as a drill or burr into the working space of pathway310 without potentially obstructing the direct visualization of thepathway 310 by the surgeon. In addition, depth stops may be added toinstruments 322 placed down angular side access channel 320 for greatercontrol of penetration depth while removing soft tissue and bone priorto graft delivery. The top view of spine 20 as illustrated in FIG. 7demonstrates placement of the instrument tip 322 a across screwextension 14 and into the operative pathway 310 provided by tubularretractor 300. In this embodiment it is also possible to add a depthstop across the extension slot in order to limit the depth ofpenetration of the powered instrument. Multiple tissue preparationinstruments for preparing bony surfaces or soft tissue for may be placedsimultaneously through the three different access openings provided bypathway 310, windows 314 and angular access channel 320.

Referring now to FIGS. 8-10, a fourth alternative tubular retractor 400for use in minimally invasive spinal fusion system 1 is described.Tubular retractor 400 includes a modification of tubular retractor 200.Reference numerals illustrating elements of tubular retractor 400 thatare common with tubular retractor 200 are increased by 200 for ease ofdescription. In tubular retractor 400, an angular window guide funnel418 having a guide channel 420 communicates with one of windows 414. Theconfiguration of tubular retractor 400 may facilitate a controlledplacement of a blunt dilator 422 by an introducer instrument 424 asshown in FIG. 9 capable of creating a subcutaneous tissue pathwaybetween two spinal vertebral bodies as shown in FIG. 10 prior to graftinsertion. The shape of guide funnel 418 may assist in reducing tissuemigration into window 414.

Turning now to FIG. 11, a fifth alternative tubular retractor 500 foruse in minimally invasive spinal fusion system 1 is described. Tubularretractor 500 includes a modification of tubular retractor 200.Reference numerals illustrating elements of tubular retractor 500 thatare common with tubular retractor 200 are increased by 300 for ease ofdescription. Tubular retractor 500 may be used for placement of a bonegraft material between adjacent spinal levels. An opening 514 a isprovided at distal end of windows 514. These respective openings 514 aallow for initially placing a bone graft material in a generally 90°orientation (relative to the skin line S) and then rotating the graftmaterial up to about 90° into position between the distal forks 502 aand 502 b on either side of opening 514 a prior to deploying the graftmaterial between the two spine segments.

Having described various alternative tubular retractors, a method ofusing such tubular retractors in minimally invasive spinal fusion system1 is now set forth, with reference primarily, but not exclusively, totubular retractor 200 and FIGS. 12-21. The primary anchor point for thevarious steps involved in the access, visualization, preparation, andgrafting procedure is based on the fixation of multi-axial pedicle screw12 into the pedicle of a vertebral body. This is a well-known andestablished anchor point. Once the tubular retractor 200 is affixed tothe screw extension 14, pathway 210 is laterally offset from screwextension 14 and rotatable and articulatable with screw extension 14about spinal screw shaft 12 a. The surgeon is therefore able to relyupon a fixed access point that is stable and well known anatomically toestablish access to bony surfaces of spinal elements surrounding pediclescrew 12. In addition, the multi-axial feature of pedicle screw 14allows for multiple angles of visualization, preparation, and graftplacement as the tubular retractor 200 is rotated and pivoted about thescrew shaft 12 a.

The initial step of the procedure involves the formation of smallincision I₁ through the skin line S and into the tissue of a patient asshown in FIG. 12. The term “small incision” as used herein is meant tomean an incision of about 4 mm to 25 mm in length sufficient forintroduction of the initial dilator 34, and preferably in the range ofabout 5 mm to 10 mm. A pedicle screw extension assembly 10 is placedthrough a first incision I₁ with pedicle screw 12 threadedly attached tothe pedicle 16 of a vertebral body 18 in a manner as described in the'640 patent, incorporated herein by reference. Screw extension 14 has alength that projects outwardly from skin line S. The skin and tissuesadjacent to screw extension 14 are dilated using a series ofprogressively larger smooth dilators, the initial dilator 34 being shownin FIG. 12. The initial dilator 24 is slid axially adjacent to screwextension 14. This is followed by one or more larger dilators to providefor the exposure necessary for placement of outer tubular retractor 200.The largest dilator 36 is shown in FIG. 13.

Once the tissue has been dilated, tubular retractor 200 may be placed asis illustrated in FIG. 14. Tubular retractor 200 is slid downward inclose contact with dilator 36 and screw extension 14 and is manipulatedthrough the skin S and a muscle layers down to the posterior elements ofthe spine 20. The presence of a tab 216 allows for stabilization of thetubular retractor 200 once fully seated, but also provides the surgeonwith an easy handle for pushing down and manipulating tubular retractor200 through the soft tissues and muscle. FIG. 15 shows the sequentialarrangement from the pedicle screw 12 fixed to the vertebral bodythrough the screw extension 14 and tubular retractor 200 and dilators 34and 36.

At completion as shown in FIG. 16, tubular retractor 200 is placed fullydown and seated against the posterior elements of spine 20. At thispoint, dilators 34 and 36 are removed and an access channel is provideddirectly to the posterior elements through pathway 210. During placementof tubular retractor 200 over dilator 36 and pedicle screw extension 14flat interior surfaces 204 c of attachment portion 204 engage one ofopposing flat surfaces 24 on screw extension 14 thereby keying tubularretractor 200 to screw extension 14 substantially preventing relativerotation therebetween. Because screw extension 14 is attached tomultiaxial pedicle screw 12, tubular retractor 200 may be rotated andpivoted about screw shaft 12 a in order to allow direct visualizationand access to the midline elements of spine 20, such as the facet andpars intra-articularis in the medial direction. In addition, tubularretractor 200 may be rotated laterally for direct visualization of theinter-transverse processes.

Multiple tubular retractors 200 may be placed in order to provide accessto the posterior spinal elements across one or more spinal segments. Asshown, for example in FIG. 16, a second small incision I₂ may be madethrough the skin line S and into the tissue of a patient at an adjacentvertebral level of spine 20. A second pedicle screw extension assembly10 may be placed through incision I₂ with pedicle screw 12 threadedlyattached to the pedicle 16 of the adjacent vertebral body 18 in the samemanner as the attachment of the first pedicle screw extension assembly10. Likewise a second tubular retractor 200 as shown in FIG. 17 may beslid downward in close contact with a dilator 36 and screw extension 14and manipulated through the skin S and muscle layers down to theposterior elements of the spine 20 as with first tubular retractor 200.Different sized tubular attractors 200 are shown placed in the regionsadjacent to the posterior spinal elements. A top view as shown in FIG.18 demonstrates the visualization of the spinal elements through thetubular retractors 200 as would be seen by the surgeon. There aremultiple sizes of tubular retractors 200 that would be available basedon the working space required and the anatomy of the patient andthickness of overlying soft tissues of muscle in the region.

Once tubular retractor 200 has been placed and a visualization pathwayis provided down to the posterior spinal elements of spine 20, the nextstep in the procedure is to remove soft tissues overlying the bonystructures. FIG. 19 shows use of an electrocautery device 38 for removalof overlying soft tissues and blood vessel cauterization with the tip ofelectrocautery device placed directly through side window 214. The softtissues must be removed prior to bony preparation necessary for securingbleeding decorticated bone along the spinal elements required forachieving spinal fusion. While an electrocautery device 38 is shown,other instruments such as rongeurs and curetttes may also be used toefficiently remove overlying soft tissues under direct visualization bythe surgeon.

There are multiple means by which the surgeon may choose to visualizethe working space though pathway 210 and to provide for access by theinstruments for soft tissue removal and bony preparation. For example,an operative microscope may prove useful for visualization andnavigation during the surgical procedure. A surgeon may utilizeoperative loops that provide for magnification and direct illuminationby way of a light source that is fixed to the surgeon's forehead.Finally, a separate endoscopic camera system may be used forillumination and visualization, where the small tubular endoscope wouldbe placed down the working pathway 210 and visualized on a separatemonitor within the operating suite.

Instruments may be placed by way of a number of access corridors. FIG.20 shows a powered drill 40 or burr for controlled preparation andremoval of the bone. Drill 40 is placed directly through pathway 210. InFIG. 21 drill 40 is placed through one of the side windows 214 while thesurgeon directly visualizes its operation through the pathway 210 oftubular retractor 200.

One of the primary advantages of system 1 is the ability to balancecontrol with a wide range of orientation and access corridors foraddressing the adjacent bony elements, both medial (facet, parsinterarticularis, lamina) and lateral (transverse processes). Havingmultiple degrees of freedom provided by the multiaxial pedicle screw 12provides the surgeon with variations in orientation that are desirableto adequately decorticate the surrounding bony elements prior to graftplacement for spinal fusion.

Once the adjacent soft tissues have been removed and the posterior bonyelements have been decorticated to expose bleeding bone, the next stepis for the surgeon to deposit a suitable bone graft on the prepared bonysurfaces. In some cases, it may be adequate simply to place bone graftaround the decorticated facet joint to fuse the facet joint. Bone graftmaterial may be placed through a side opening, such as one of windows214, or through pathway 210 of tubular 200. Facet fusion can provide forsome enhanced stability alone or in combination with an interbody spinalfusion. However, in many cases the facet joint on one side of the spinewill have been removed in order to allow for passage of an interbodyimplant. Fusion of the contralateral facet joint alone may provide somebenefit, but creation of a bridging bony fusion across two spinalsegments on both sides of the spine provides for a more robust andconventional posterior lateral spinal fusion.

In the case where such as a bridging fusion is desired, a dilator, suchas blunt dilator 422 described hereinabove with reference to FIGS. 9-10may be affixed to the an introducer, such as introducer instrument 424to create a subcutaneous passageway through tissue between the twospinal segments. FIG. 22 shows the placement of blunt dilator 422through side opening window 214 of tubular retractor 200. It should beappreciated, however, that tubular retractors 400 and 500 as describedhereinabove may also be used in the placement of blunt dilator 422 forcreating the subcutaneous passageway. Blunt dilator 422 may be placedunder direct visualization and then pushed through the soft tissuepassageway until it extends into window 214 of the second tubularretractor 200.

FIG. 22 demonstrates the use of multiposition introducer instrument 424that allows for placement of blunt dilator 422 at multiple orientations.Initially, blunt dilator 422 may be placed in a nearly verticalorientation through incision I₁ down along the sides of the screwextension 14 and into window 214 of tubular retractor 200. Once bluntdilator 422 contacts the posterior bony elements of spine 20 it may berotated into a 90 degree orientation and pushed subcutaneously acrossthe passageway between the two spinal elements. Multiple sizes of bluntdilators 422 may be used in a progressive manner to safely and gentlydilate the tissues between adjacent spinal levels.

Following creation of the tissue passageway between spinal segments,there are number of graft material options that could be useful forbridging the spinal elements, such as those shown in FIGS. 23-31. Asdescribed further below, these graft material options include acylindrical and pliable bone or synthetic graft material tube. Thecylindrical graft may be solid, hollow or partially open and can be usedto contain an injected graft material, for example. Graft implantdesigns may also include a more rigid cortical bone, a pliablecancellous bone plug, or synthetic graft material shape to mimic thespinal curvature and provided with a channel and outlets for injectionof flowable graft material two fully fill the variable anatomy aroundthe graft implant. Further, a series of sleeves that are pliable andperforated could be placed in a manner analogous to placement of acardiovascular or urologic stent.

FIG. 23 shows placement of a hollow perforated graft 42 that may allowfor a bony incorporation and act as a pathway and scaffold for placementof additional fusion-facilitating materials. Alternatively, a perforatedtube 44 could be placed as shown in FIG. 24 (without showing retractors200 for clarity) allowing for injection of a biomaterial capable ofsecuring bridging spinal fusion. This perforated tube 44 may be removedafter placement of the fusion material. Once the tissue passageway isbridged with tube 44, a long cannula 46 may be used for injection of aflowable bone graft substitute. Such flowable bone graft material mayinclude a synthetic hardening bone graft substitute that would set andprovide for the appropriate rigidity and stabilization to secure spinalfusion.

A manufactured perforated anatomical implant 48 is shown in FIG. 25.Implant 48 may be fabricated from metal, polymer, ceramic, or a naturalmaterial such as allograft bone. Implant 48 would help to maintain thetissue passageway previously created, while providing a scaffold ontowhich and into which injectable biomaterial could be delivered.Following placement of implant 48 across the spinal segment, additionalspinal fusion-enabling material 50 may be placed with directvisualization by an injector 52 through pathway 210 of tubular retractor200. FIG. 25 demonstrates this method of perforated implant deliveryfollowed by flowable biomaterial injection. The flowable biomaterialcould be one of many options available to surgeons. It may involve asetting artificial ceramic material, or could involve the use of aslurry of morselized autograft or allograft or bone marrow aspiratecontaining mesenchymal stem cells, all capable of injection by way ofrelatively small diameter cannula 54, as shown.

Alternatively, a flexible and perforated tubular implant 56, as shown inFIGS. 26-27 may be used. This implant could be a synthetic biomaterialor an allograft or autograft plug of cancellous bone. Demineralizationof the bone would render it more pliable if needed for insertion downthe cannula. In this case, implant 56 is pushed along a curved rigidaccess tube 58 into the space between the spinal levels. Access tube 58is placed in the space between the spinal elements as visualized throughpathways 210. Curved access tube 58 allows for deposition of implant 56through a trajectory necessary for bridging spinal elements adjacent twospinal screws 12, as depicted in FIG. 27 showing implant 56 in theprocess of being delivered to the spine. A flexible tamp 60 is used topush flexible implant 56 into place. Flexible implant 56 may befabricated, for example, of a partially demineralized cancellous(spongy) bone machined into a semicircular shape and perforated. Onceimplant is expelled from access tube 58 and is in place flowablebiomaterial may be injected. FIG. 28 shows additional bone graftmaterial (such as a ceramic hardening cement or bone marrow aspirate)being injected by injector 52 around implant 56 for an enhanced fusionbed as well as better anchorage of implant 56 at its two opposite ends56 a and 56 b. FIG. 29 shows some of the injected graft material 62surrounding the previously placed implant 56. Biomaterial 62 may beinjected into and around the perforated implant 56 as shown in FIG. 29to provide for more surface area, more graft volume, and enhancedanchorage of the implant in the region in which the posterolateralspinal fusion is desired.

Turning now to FIGS. 30-35 an alternative technique for placing a graftimplant such as flexible implant 56 is described, with particularreference to tubular retractor 500, as described hereinabove withrespect to FIG. 11. As described, tubular retractor 500 may be used forplacement of implant 56 between adjacent spinal levels. Opening 514 a atthe distal end 500 a of tubular retractor 500 allows for initiallyplacing implant 56 in a 90° orientation (to the skin line S) and thenrotating it 90° into position between the distal forks 502 a and 502 band pushing it down to the prepared bony surfaces through openings 514 ain a manner analogous to the minimally invasive insertion of theconnecting rod described in the '640 patent.

A self-contained delivery device 64 for holding and delivering implant56 is shown in FIG. 30. Device 64 allows for preloading implant 56 (notshown) into a delivery device outer sheath 66 through an opening 68 atthe distal end 66 a. Once implant 56 has been loaded the implant 56 maybe placed in position between two spinal elements and a plunger 70 atthe proximal end 66 b is pushed downwardly in order to displace theimplant 56 into the previously created tissue passageway between thevertebral elements.

A delivery device 72 for more controlled delivery of implant 56 is shownin FIG. 31. Device 72 like device 64 allows for preloading implant 56(not shown) into a delivery device outer sheath 74 through an opening 76at the distal end 74 a. Distal end 74 a is configured and sized toextend into windows 514 and across tubular retractor 500, as shown inFIG. 32. Once implant 56 has been loaded into outer sheath 74, distalend 74 a may be placed through windows 514 for positioning implant 56between two spinal elements. In this case, a rather large handle 78 isprovided for the surgeon for stabilization and orientation of deliverydevice 72. A screw thread 80 allows for controlled withdrawal of acollar 82 that will draw outer sheath 74 back and expose implant 56 onceit has been docked into the desired location between the spinalelements.

Rather than pushing implant 56 out into the space in which bone graft isdesired, delivery device 72 allows the surgeon to essentially “park”implant 56 across the elements in the location desired and thengradually withdraw outer sheath 74 relative to handle 78, leavingimplant 56 behind. This process is shown in FIGS. 33-34 prior towithdrawal of outer sheath 74. In FIG. 34 outer sheath 74 has beenwithdrawn completely leaving implant 56 behind. As depicted in FIG. 34,delivery device 74 is constructed to include an inner access tube 84which remains stationary relative to handle 78 while outer sheath 74 iswithdrawn upwardly over tube 84. Implant 56 engages inner tube 84 and isthereby maintained in place during the outer sheath withdrawal process.FIG. 35 shows implant 56 in enlarged view after being pushed downthrough opening 514 a of window 514 of tubular retractor 500 towardspinal elements. The dimensions of implant 56 are selected by thesurgeon according the anatomy of the patient to span the spinal segment.In the lower lumbar spine, for example, the length of implant 56 may beas large as 60 mm, while in the upper thoracic spine the lengths may bein the range of about 20 to 25 mm. Diameter will range from 5 mm to 15mm with a diameter of about 9 mm being preferred.

It should now be appreciated that a bone graft material such as bonegraft implant 56 can be placed percutaneously across a spinal segment byspinal fusion system 1 as described herein in order to fuse two a morevertebral bodies of spine 20. The placement of graft implant 56 on thetransverse processes 86 and 88 of respective vertebral bodies 90 and 92is illustrated in FIG. 36. Using a pair of pedicle screw extensionassemblies 10 and a tubular retractor 500 together with various tissuepreparation instruments as described herein, graft implant 56 has beenplaced in the position shown in FIG. 36 through a small incision, suchas incision I₁ or I₂, and then moved subcutaneously through a tissuepassageway formed by a blunt dilator 422 adjacent to pedicle screws 12previously inserted into the pedicles of vertebral bodies 90 and 92. Inthe case depicted in FIG. 36, implant 56 is disposed laterally ofpedicle screws 12 on transverse processes 86 and 88. With theinstruments of spinal fusion system 1 and related instruments andmethods of use as described herein, implant 56 may have been placedalternatively more midline, such as on the elements of the facet or parsintra-articularis.

With graft implant 56 suitably placed in the desired position on thespinal elements and with the tubular retractor 500 removed from screwextension 14 of pedicle screw extension assembly 10, suitable fixationmay also be implanted percutaneously adjacent to graft implant 56. Usingslots 26 of screw extensions 14 respectively projecting outwardly fromthe patient through incisions I₁ and I₂ a connecting rod 94 may beinstalled in a manner as described in the '640 patent. In suchprocedure, connecting rod 94 is pivotally attached at its proximal end94 a to a suitable rod introducer (not shown). The distal end 94 b ofconnecting rod 94 is inserted through incision I₁ or I₂ and throughslots 26 of a first pedicle extension 14 and is then movedsubcutaneously through tissue and into and through slots 26 of a secondpedicle extension 14. Connecting rod 94 is suitably disconnected fromthe rod introducer, screw extensions 14 are removed from pedicle screws12 and incisions I₁ and I₂ are suitably sutured to complete this aspectof the surgical procedure. Another graft implant 56 and connecting rod94 may be installed on the contralateral side of spine 20 to vertebralbodies 90 and 92 in the same manner as the installation of first graftimplant 56 and connecting rod 94. It can thus be appreciated that theentire surgical procedure including the implantation of graft materialto promote fusion between vertebral bodies 90 and 92 and associatedfixation may be performed percutaneously.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. For example, while theplacement of the graft implant as well as associated fixation can beplaced on the spinal elements in a percutaneous method whereby the graftimplants and connecting rods are placed through individual separateincisions, it can be appreciated that the spinal fusion system andinstruments described herein may also be used minimally invasively. Assuch, in situations for instance where the vertebral bodies of theaffected spinal segment are relatively close the formation of separateincisions associated with each vertebral body may be difficult, if notimpossible, to achieve. In this instance, a single incision, which maybe longer than the small incision described hereinabove, may be formedfor access to both vertebral bodies with two pedicle screw extensionassemblies and tubular retractors introduced through the same incisionin a minimally invasive procedure. In some situations, a surgeon mayfind it advantageous to use the system described herein, or certain ofthe instruments or tubular retractors for an open spinal procedure.

A further variation within the contemplated scope of the invention isshown in FIGS. 37-39. A primary focus of the spinal fusion system 1described heretofore rests on utilization of multi-axial pedicle screw12 as a fixed access point which allows for simultaneous rotationalvariability of screw extension 14 and an attached tubular retractor,such as tubular retractor 100, 200, 300, 400 or 500. Once themulti-axial screw 12 is placed in the pedicle, it is possible that thescrew head could obscure the surgeon's access to some areas of thespinal elements, such as the transverse processes in which there may beinterest in removing bone and providing for a bony surface for spinalfusion. FIG. 37 shows the use of a rigid tap 98 as an alternative accesspoint, where tap 98 would be placed prior to placement of multi-axialscrew 12. Tap 98 is an elongate fixation member including a distal end98 a and a proximal end 98 b. Distal end 98 a is threaded fixationelement for insertion into a pedicle of a vertebral body. An elongateextension 98 c extends proximally from distal end 98 a defining alongitudinal axis 98 d along the length of extension 98 c. Threaded end98 a and elongate extension 98 c are fixed together for joint rotation.

It is noted that rigid tap 98 would not provide for the multi axialdegrees of freedom and rotation provided by multi-axial screw 12. Assuch, a modified retractor 600 as depicted in FIG. 38 is contemplated.Tubular retractor 600 includes a modification of tubular retractor 200.Reference numerals illustrating elements of tubular retractor 600 thatare common with tubular retractor 200 are increased by 400 for ease ofdescription. Unlike tubular retractor 200, however, attachment portion604 of tubular retractor 600 does not have a flat interior surface andthereby is not keyed to extension 98 c of tap 98 when tubular retractor600 is releasably attached to extension 98 c.

Upon attachment of attachment portion 604 to extension 98 c thelongitudinal attachment axis of attachment portion is substantiallycoaxial with longitudinal axis 98 d of extension 98 c and pathway 610 islaterally offset elongate extension 98 c. While threaded end 98 a andextension 98 c are rotationally fixed, attachment portion 604 andthereby tubular retractor 600 with pathway 610 may freely rotate aboutextension 98 c. Rotation of pathway 610 would allow for a broader areaof bony decortication that could potentially be impeded by the presenceof the head of pedicle screw 12 introduced later in the procedure. Inparticular, rotating tubular retractor 600 laterally allows for directvisualization of the inter-transverse process and full decortication ofthose elements prior to bridging bone graft placement. However in thiscase, the primary exposure and access needed via tap 98 is to providefor decortication of the transverse process 32 of spine 20. FIG. 39shows an overview of tap 98 and tubular retractor 600 placed over thetap extension 98 c. Access port pathway 610 provides for orientation andaccess to transverse process 32.

It should therefore be understood that only the preferred embodimentshave been presented and that all changes, modifications and furtherapplications that come within the spirit of the invention set forth inthe claims are desired to be protected.

What is claimed is:
 1. A system for minimally invasive spinal fusion,comprising: a pedicle screw extension assembly comprising a multi-axialpedicle screw and an elongate screw extension, said multi-axial screwincluding a threaded shaft having a yoke articulatingly and rotatablyattached to said screw shaft, said elongate screw extension beingreleasably coupled to said yoke of said screw for articulation androtation of said extension about said screw shaft, said elongateextension defining a first longitudinal axis along the length of saidextension; and a tubular retractor releasably attached to said extensionfor joint articulation and rotation with said extension about said screwshaft, said tubular retractor including an access port having aperimetric wall defining a pathway therethrough configured for accessand visualization, said pathway having a second longitudinal axis offsetfrom said first longitudinal axis of said screw extension, saidperimetric wall enclosing said pathway at least half-way around saidsecond longitudinal axis, said tubular retractor comprising anattachment portion monolithically joined with and offset from saidaccess port, said attachment portion defining a third longitudinal axis,said third longitudinal axis being coaxial with said first longitudinalaxis of said screw extension, said attachment portion of said tubularretractor being cooperatively keyed to said pedicle screw extension in amanner to prevent relative rotation therebetween and allow relativeaxial movement therebetween, said tubular retractor having a distal endand a proximal end, said access port having a length along said secondlongitudinal axis and said attachment portion having a length along saidthird longitudinal axis, the length of said access port and saidattachment portion being different, said attachment portion extendinglongitudinally from said distal end to said proximal end, and saidaccess port extending from said distal end for a length along saidtubular retractor toward said proximal end, the length of said accessport being less than the length of said attachment portion.
 2. Thesystem of claim 1, wherein said access port extends longitudinally fromsaid distal end to said proximal end, said attachment portion comprisinga first attachment element adjacent said distal end and a secondattachment element adjacent said proximal end, said first attachmentelement being longitudinally spaced from said second attachment element.3. The system of claim 2, wherein the longitudinal space between saidfirst attachment element and said second attachment element defines aside opening in communication with said pathway.
 4. A system forminimally invasive spinal fusion, comprising: a pedicle screw extensionassembly comprising a multi-axial pedicle screw and an elongate screwextension, said multi-axial screw including a threaded shaft having ayoke articulatingly and rotatably attached to said screw shaft, saidelongate screw extension being releasably coupled to said yoke of saidscrew for articulation and rotation of said extension about said screwshaft, said elongate extension defining a first longitudinal axis alongthe length of said extension; and a tubular retractor releasablyattached to said extension for joint articulation and rotation with saidextension about said screw shaft, said tubular retractor having a distalend and a proximal end and including an access port having a perimetricwall defining a pathway therethrough configured for access andvisualization, said pathway having a second longitudinal axis offsetfrom said first longitudinal axis of said screw extension, saidperimetric wall enclosing said pathway at least half-way around saidsecond longitudinal axis, said tubular retractor comprising anattachment portion monolithically joined with and offset from saidaccess port, said attachment portion defining a third longitudinal axis,said third longitudinal axis being coaxial with said first longitudinalaxis of said screw extension, wherein said attachment portion extendslongitudinally from said distal end to said proximal end and said accessport extends from said distal end for a length along said tubularretractor toward said proximal end, the length of said access port beingless than the length of said attachment portion.
 5. The system of claim4, wherein said attachment portion comprises a curved open channel andwherein said wall of said access port substantially encloses saidpathway.
 6. The system of claim 5, wherein said access port has a firstside opening defined by a first window extending through said wall incommunication with said pathway, said first window being configured forreceipt of a tissue preparation instrument and delivery of bone graftmaterial.
 7. The system of claim 6, wherein said access port has asecond side opening defined by a second window extending through saidwall in communication with said pathway.
 8. The system of claim 7,wherein said access port further includes an angular window guidechannel communicating with said first window.
 9. The system of claim 7,wherein said attachment portion includes a side opening defined by anangular side access port having a guide channel extending through andintersecting said third axis.
 10. The system of claim 9, wherein saidelongate screw extension has a lumen extending longitudinally from saiddistal end to said proximal end and a pair of opposed longitudinal slotsextending through said extension in communication with said lumen, saidangular side access guide channel communicating through said slots andsaid lumen of said extension with said pathway of said access port.